The present invention relates to a system for controlling active suspensions of a vehicle.
Active suspension systems of various forms have been developed and publicly disclosed as in Japanese Pat. Appln. Laid-Open Publn. No. 62-139709. A typical example of a known active suspension system comprises the following essential components. Individual suspensions are provided for each wheel for supporting the vehicle by fluid pressure. Charging and discharging of fluid into or out of each suspension is controlled independently by the operation of a respective control valve. Each control valve is controlled by opening and closing control signals generated by a controller responsive to information such as vertical acceleration of the vehicle mass above each suspension unit and vertical displacement of the suspension. The controller calculates command quantity of charging or discharging of the fluid for each suspension unit. Thus the charging and discharging of the fluid into or out of each suspension is controlled.
Applicant has previously developed an active suspension system described below and has filed a U.S. patent application therefor (U.S. application Ser. No. 410,834 now patent No. 5,033,770). In the active suspension system as described above, sensors are used to detect accelerations in the longitudinal and lateral directions of the vehicle. The above described controller preestimates variations of the vehicle attitude (pitching and rolling) accompanying acceleration, deceleration or turning of the vehicle in response to the information from the sensors. Then, the controller computes the command quantity of the charge and discharge of the fluid for maintaining the vehicle attitude in a desirable state, thus generating and transmitting signals for opening and closing the control valves.
Applicant has also developed another active suspension system and filed a U.S. patent application (U.S. application Ser. No. 425,961 now patent No. 5,029,328). The system has means for controlling the vehicle attitude based on the longitudinal and lateral acceleration of the vehicle, as well as means for arbitrarily selecting and setting a vehicle rolling direction and a rolling degree during turning thereof according to a preference of the driver.
In the active suspension system controlling the vehicle rolling based on the lateral acceleration (lateral G), as described, when the lateral G becomes large, the quantity of lateral load shift occurring with the lateral G becomes large, and the suspension reaction force required for maintaining the rolling angle at zero or at a desired angle also becomes large. For this reason, when the lateral G approaches an upper limit value (approximately 0.9 G for normal passenger cars), the lateral load shift quantity becomes extremely large and it becomes difficult to maintain a desired vehicle attitude and to carry out control for improvement of the drive sensation in that state.
For example, a vehicle will be considered in which a lateral load shift as indicated in FIG. 4 occurs at the right front wheel of the vehicle due to the lateral G. When it is desired to control the rolling of the vehicle within the range of from (-0.9G) to (+0.9G), the control system requires the capability of controlling the suspension reaction force in the range of from (-x) to (2W.sub.o +X)kgf (where W.sub.o is an initial suspension reaction force for the right front wheel). In order to produce a suspension reaction force of (-x)kgf, a special suspension unit of a double-acting opposite-piston type is required. Furthermore, in order to produce a force of twice the initial suspension reaction force W.sub.o, an initial pressure (standard pressure) of the suspension units must not exceed half the internal pressure of a high-pressure pump in the case where a hydro-pneumatic type suspension is used. This means that a high performance is required for the high pressure pump.